At present, a representative example of a method of manufacturing a seamless steel pipe (referred to below as pipe manufacture) is a method in which piercing with skewed rolls (referred to below as piercing) is performed on a billet using a piercer (a piercer with skewed rolls) to obtain a hollow tube shell (referred to below as a shell). The shell is rolled and elongated by a rolling mill such as an elongator, a plug mill, or a mandrel mill, and then finally it is sized by a sizer or a stretch reducer.
In this case, if the material for forming the seamless steel pipe is an ordinary low carbon steel having a relatively low content of alloying components, it is relatively easy to obtain a good quality shell using a piercer, which is advantageous from the standpoint of mass production. However, when using a high alloy steel such as SUS316, SUS321, or SUS347 specified by JIS or other austenitic stainless steel as a material to be worked, since these materials are difficult to work materials, if a piercer is employed, inner surface flaws caused by Mannesmann breakdown which is characteristic of piercing can easily form in the shell, and if inner surface flaws form, there are cases in which it becomes impossible to obtain a good quality seamless steel pipe.
There has yet to be reported a suitable measure for preventing the formation of inner surface cracks which can be applied to actual production lines. Therefore, it has been thought difficult to carry out mass production on an industrial scale of seamless steel pipe of a high alloy steel such as an austenitic stainless steel.
In addition to the above, in the case of an austenitic stainless steel, inner surface flaws caused by grain boundary melting can easily occur. Grain boundary melting is a phenomenon in which low melting point substances present at grain boundaries melt due to the heat generated by working in a piercer with skewed rolls. If grain boundary melting occurs, the ductility of the material abruptly decreases, and this leads to breakage, i.e., cracks at the time of piercing of the shell.
The above-described grain boundary melting occurs in the body of a material including the inner surface thereof where the temperature of the material becomes highest during piercing. Flaws which propagate from there as a starting point are almost impossible to repair, and so this unavoidably leads to a marked decrease in yield.
With austenitic stainless steel and particularly austenitic stainless steels such as SUS316, SUS321, and SUS347 which contain alloying elements such as Mo, Ti, Nb, and Cu, these alloying elements easily form low melting point substances, so grain boundary melting occurs particularly readily. In addition, if these alloying elements are added, the strength of the material increases, and the heat generated by working during piercing increases, and this becomes a cause of promoting the occurrence of grain boundary melting.
In order to prevent this grain boundary melting, it is thought that piercing which suppresses the heat generated by working with a piercer is effective.
In order to carry out piercing while suppressing the heat generated by working, normally, a method is employed in which the rotational speed of skewed rolls is decreased and the strain rate of the material is decreased, or a method in which the wall thickness of the pierced material is increased.
However, if the rotational speed of the rolls is decreased, time is required for piercing with the piercer, and not only is the lifespan of tools (particularly a plug) greatly decreased, but the temperature of the resulting shell decreases, so a method in which the rotational speed of the rolls is decreased, i.e., a method in which the speed of piercing is decreased cannot be applied to an actual production line.
If the wall thickness of the material being pierced is increased, rolling performed in a pipe rolling mill (such as an elongator, a plug mill, or a mandrel mill) disposed downstream of the piercer becomes unstable, and the manufacturing yield of seamless steel pipes enormously decreases, so this method, too, can not be applied to an actual production line.
In order to stabilize rolling in a pipe rolling mill disposed downstream of a piercer, it is desirable to supply the rolling mill with a thin-walled material which is at as high a temperature as possible, i.e., a thin-walled shell at a high temperature. However, if the heating temperature of a billet to be worked is increased in order to supply a high temperature shell, the material reaches a temperature at which grain boundary melting occurs with even a slight amount of heat generated by working, so it was all the more difficult to carry out piercing to form a thin wall thickness which requires a large degree of working under conditions in which the heating temperature of a billet is increased in this manner.
In Japanese Published Unexamined Patent Application 2000-301212, as a method for piercing a difficult to work metal, a piercing method in which the heating temperature of a billet and the speed of piercing by a piercer are adjusted in conjunction with each other, and as a result the temperature of the billet is maintained at lower than an overheating temperature (1260-1310° C.), and piercing is performed is disclosed. Here, the overheating temperature is a temperature which brings about grain boundary melting of the material. The grain boundary melting temperature for a austenitic stainless steel such as SUS316, SUS321, and SUS347 is in the range of 1260-1310° C.
However, the method disclosed in Japanese Published Unexamined Patent Application 2000-301212 merely controls the value of a formula using the piercing speed and billet heating temperature as variables to less than the overheating temperature and thereby aims at preventing the billet temperature during piercing from being greater than or equal to the overheating temperature. From the examples thereof, specifically, it can be seen that in order to obtain a shell without flaws, it is necessary to heat the billet to a low temperature of 1100-1180° C.
In addition, in the examples of the above-mentioned publication, the piercing speed is at most 300 mm/second, so when obtaining a shell with a length of 8 m, 30 seconds are required, which is not practical.
Furthermore, in the examples, a simulation of plasiticine is carried out. At that time, the ratio (the t/d ratio) of the wall thickness to the outer diameter of the shell after piercing is 15%, so the wall thickness is considerable.
Accordingly, with this method, stable rolling cannot be guaranteed in the subsequent rolling mill, and in addition, the lifespan of the piercer tool is not adequate.
In “CAMP-ISIJ”, Volume 6 (1993), pages 370-373, an example is reported of piercing of SUS316L with a piercer in an actual production line. However, in that report, in order to prevent flaws on the inner surface of the pierced shell, it is necessary to decrease the peripheral speed of skewed rolls as well as to control the heating temperature of a billet to at most 1190° C. so there are problems like those of the method disclosed in the above-mentioned Japanese Published Unexamined Patent Application 2000-301212.
Japanese Published Unexamined Patent Application 2001-162306 discloses a method of preventing flaws on the inner surface of a pierced shell by controlling the value of a formula using the billet diameter, the diameter of skewed rolls, and the rotational speed of skewed rolls as variables. However, with this method as well, piercing is carried out while reducing the rotational roll speed of skewed rolls, and it is essentially no more than a means of limiting the piercing speed, i.e., the strain rate of the material, and it has problems such as an increase in the time required for piercing, a decrease in tool lifespan, and a decrease in the temperature of a shell, so it cannot be said to be a means which can be applied to an actual production line.